There have been developed recording or printing materials in which an image formed by the material may be erased by applying heat over a prescribed temperature to an image formed of the recording material. When such recording material is used to print on a sheet of paper, by applying heat on the image printed on the paper sheet, the paper sheet can be reused. However, in order to reuse the paper sheet, the color of the image should be erased to a state over a prescribed level of thoroughness, i.e., to a level at which the erased image is not readily apparent to the naked eye, and the paper sheet should not be damaged. In recent years, there has been developed a decoloring system with the function of erasing the color (including black and when on a colored paper sheet, white) formed on the paper sheet and the function of determination of whether the paper sheet can be reused.